This invention relates to electrophoretic liquid development and more particularly to an improved method of plural stage development.
In the liquid development of electrostatic charge latent images, as in electrophotography and in other processes that form and develop electrostatic charge patterns, a substrate having a charge pattern on its surface is contacted with a liquid developer which is essentially a suspension of colloidal toner particles in an insulating liquid. Liquid developers normally contain also a stabilizer or charge control agent. The latter is an ionic compound which controls the magnitude of the charge on the toner particles and aids in maintaining a stable charge on the toner particles within the insulating carrier liquid.
Liquid developers can be used in single stage or plural stage development processes. Examples of the latter may include the sequential development on a photoconductor of two or more color-separation images, the annotation of a previously developed image, or the repeated re-exposure of and development of images on a reusable photoconductor, with transfer of images after each development.
In certain plural stage development processes which use liquid developers a problem has been found which is especially significant in processes for the electrophotographic reproduction of multi-color images of graphic arts quality. In these processes electrostatic latent images are formed sequentially on a chargeable substrate such as an electrophotographic medium, with liquid development or toning of each latent image before the next is formed. A leading example of this kind of process involves exposing an electrophotographic medium sequentially through a series of four registered color-separation transparencies with four sequential development or toning stages. The problem encountered in this kind of process is that image defects occur in development of charge patterns in areas of the substrate which are background or untoned areas of a previous stage. The defect appears as an image in the previously untoned areas which is dull and has a low, non-uniform solid density, a defect which, though perhaps acceptable in some kinds of add-on, plural stage imaging, is not aceptable for producing high quality images as required, for example, in the graphic arts field.
It has been discovered that the described image defects an plural-stage liquid development appears to be caused by the presence of forces of substances in the previously untoned areas that interfere with subsequent development in those areas. Although the nature of those forces or substances is not clear, the present invention provides a method and apparatus that reduces or eliminates the problem.
The method of the invention comprises uniformly charging a photoconductive element, exposing the photoconductive element to a pattern of actinic radiation to form a latent electrostatic image, developing the latent image with a liquid developer composition comprising a carrier liquid, a toner and charge control agent, rinsing the developed surface of the photoconductive element with a rinse solution, thereafter again charging the surface of the photoconductive element, again exposing it to a pattern of actinic radiation to form a latent electrostatic image and developing the image with a liquid developer.
In a preferred embodiment, the charge control agent in the liquid developer is a polymeric organo-lithium compound, and the carrier liquid and the non-polar liquid of the rinse solution are volatile hydrocarbon liquids.
The method of the present invention is useful in any electrostatic imaging process wherein a charge pattern is formed and developed with a liquid developer on a surface which has previously been developed with a liquid developer. It is particularly useful, however, in combination with a recently developed electrophotographic method of making lithographic color proofs. This new method is described in the copending U.S. patent application of Ng et al., Ser. No. 773,528 filed Sept. 6, 1985. In this method a photoconductor which has a uniformly charged thin transparent dielectric overlayer is subjected to a series of exposures through registered color separation transparencies. After each exposure the dielectric layer is developed with a liquid developer, the surface is again uniformly charged and exposed. The sequence is repeated for each of the color transparencies, usually four.
Although we do not wish to be bound by any theoretical explanation or mechanism of the invention, a possible explanation for the results observed in the method of the invention is that the buffing of the developed substrate or photoconductor removes from untoned areas of the substrate a deposit of counterions which in some manner interfere with image development. These counterions are believed to originate from the charge control agent of the liquid developer. They are opposite in charge to the toner particles and deposit in the untoned or non-image areas of the substrate. When, in a subsequent stage, toner is applied to charge patterns in such areas, the toner is initially attracted to them. Within a short time, however, the acceptance of the toner is evidently affected because a dull, low, non-uniform solid density toned image occurs in such areas.